EventHandler: Deprecate SetUsed() and Used() as well.

Seems the latter isn't used outside of the functions that were deprecated
in the previous commit and with UsageAnalyzer not making use of this
information unclear why we should keep it around.

Relates to #3187.

src/script_opt/ScriptOpt.cc

@@ -386,7 +386,10 @@ static void use_CPP() {
 
             for ( auto& e : s->second.events ) {
                 auto h = event_registry->Register(e);
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
                 h->SetUsed();
+#pragma GCC diagnostic pop
             }
 
             auto finish = s->second.finish_init_func;